Rising temperatures threaten pollinators of fig trees—Keystone resources of tropical forests

Abstract Pollinating insects are decreasing worldwide in abundance, biomass, and species richness, affecting the plants that rely on pollinators for fruit production and seed set. Insects are often sensitive to high temperatures. The projected temperature increases may therefore severely affect plants that rely on insect pollinators. Highly specialized mutualisms are expected to be particularly vulnerable to change because they have fewer partner options should one partner become unavailable. In the highly specialized mutualism between fig trees and their pollinating fig wasp, each fig species is pollinated by only one or a few wasp species. Because of their year‐round fruit production, fig trees are considered a keystone resource for tropical forests. However, to produce fruits, wild fig trees need to be pollinated by fig wasps that typically travel a long one‐way trip from the tree donating pollen to the tree receiving pollen. In a few previous studies from China and Australia, increasing temperatures dramatically decreased fig wasp lifespan. Are these grim results generalizable to fig mutualisms globally? Here, we use survival experiments to determine the effect of increasing temperature on the lifespan of Neotropical fig wasps associated with five common Panamanian Ficus species. Experimental temperatures were based on the current daytime mean temperature of 26.8°C (2SD: 21.6–31.7°C) and the predicted local temperature increase of 1–4°C by the end of the 21st century. We found that all tested pollinator wasp species had a significantly shorter lifespan in 30, 32, 34, and 36°C compared to the current diurnal mean temperature of 26°C. At 36°C pollinator median lifespan decreased to merely 2–10 h (6%–19% of their median lifespan at 26°C). Unless wasps can adapt, such a dramatic reduction in lifespan is expected to reduce the number of pollinators that successfully disperse to flowering fig trees, and may therefore jeopardize both fruit set and eventually survival of the mutualism.

Nearly 90% of flowering plants depend on pollinators for fruit production and seed set (Ollerton et al., 2011), and the number of available pollinators directly influence the seed set of the plant (Ågren, 1996;Robertson et al., 1999). Losing pollinators therefore would have a great effect on plant communities and other organisms dependent on plants (Biesmeijer, 2006;Brosi & Briggs, 2013;Kearns et al., 1998).
Although plants themselves may tolerate rising temperatures, pollinators have quite different physiologies from plants and may be negatively affected by high temperatures (Angilletta & Angilletta, 2009;Suttle et al., 2007). Most pollinators are insects, and studies show that insects can be negatively impacted by high temperatures, having reduced lifespans, reduced fecundity, or increased thermoregulating behavior (Durak et al., 2020;Feder et al., 1997;Mech et al., 2018;Wynants et al., 2021). However, most studies on pollinators measure physiological constrains at maximum viable temperatures (Käfer et al., 2012;Maebe et al., 2021;Maia-Silva et al., 2021;Oyen & Dillon, 2018;Sánchez-Echeverría et al., 2019) rather than lifespan at elevated, realistic temperatures (Jevanandam et al., 2013;Nasir et al., 2019). Measuring the actual lifespan of pollinating insects is highly relevant as this can determine their capacity to perform pollination services. Here, we study the  (Herre, 1989;Herre et al., 2008;Wiebes, 1979).  (Herre et al., 2008).  (Dunn et al., 2008;Harrison, 2003;Jandér et al., 2016;Nason et al., 1998). Most fig wasp species that pollinate large rainforest trees disperse passively with the wind above the canopy during the day, exposing them to full sunlight and daytime temperatures (Harrison, 2003;Nason et al., 1998;. Additionally, as tropical insects, they are expected to be more sensitive to temperature increases because they are likely to already perform close to their thermal maximum (Sunday et al., 2014). During their larval development, fig wasps are sheltered within fig fruits that, at least in some species, are actively cooled by the tree (Patiño et al., 1994). However, when the adult wasps leave  Gigante et al., 2020;Jevanandam et al., 2013;Sutton et al., 2018). Are these worrying findings generalizable also   (Croat, 1978;Cruaud et al., 2012). Specifically, we studied Pegoscapus tonduzi (Ficus citrifolia), P. hoffmeyeri A and B (F. obtusifolia), P. gemellus A and B (F. popenoei), Tetrapus costaricanus (F. insipida), and T. americanus (F. maxima) (Molbo et al., 2004;Wiebes, 1995). F. obtusifolia and F. popenoei are each pollinated by two cryptic fig wasp species that cannot be distinguished morphologically (Molbo et al., 2004;Satler et al., 2020). For each fig species, one of the cryptic pollinator species is much more common than the other, and the wasps are sister species (P. hoffmeyeri A and B) or closely related (P. gemellus A and B) (Molbo et al., 2004;Satler et al., 2020) so in this study, we did not try to separate them. Non-pollinating parasitic wasps were found on all fig species but only in two fig species were there a sufficient number of parasitic wasps in our samples to include them in the statistical analyses; F. popenoei: Heterandrium spp. and Idarnes "sensu stricto" (Idarnes carme spp. and Idarnes flavicollis spp. were combined and included; Idarnes incerta spp. were not included); and F. insipida: Critogaster spp. (Bouček, 1993;Cruaud et al., 2011;West & Herre, 1994) (see Appendix 1, Tables A1-A3 for an overview of the study species and sample sizes). Parasitic wasps were identified to genus level (Bouček, 1993). Lifespans and temperature responses of the different species within each genus might differ, so the results for the parasitic wasps should be interpreted with caution.

| Study species
Additional information about the biology of the parasitic wasp genera is included in Appendix 1, part A4.

| Survival experiment
To test the fitness effect of a temperature increase we quantified the lifespan of the wasps at 26, 28, 30, 32, 34, and 36°C (except for P. hoffmeyeri [F. obtusifolia] where the highest temperature tested was 34°C). We selected 26°C as the baseline temperature in the survival models because the mean diurnal temperature of the air above the F I G U R E 2 Current and projected temperatures above the canopy at Barro Colorado Island. Temperatures were measured every 15 min at 48 m above the ground in the forest of Barro Colorado Island, Panama (Paton, 2020). The mean ± 1 SD and 2 SD are indicated.   48 m canopy at BCI is around 26°C (Paton, 2020). The experimental temperatures 26, 28, and 30°C fall within one standard deviation of the current diurnal temperatures ( Figure 2). The experimental temperatures 32, 34, and 36°C reflect the regionally projected temperature increase scenarios of 1-4°C by the end of the 21st century (IPCC, 2021). The daytime relative humidity of the air above the canopy at BCI ranges between 83% and 94% with higher values during the wet season (Paton, 2020). In our survival experiments, we used a constant relative humidity of 85% to mimic local natural conditions, and because low relative humidity can decrease wasp lifespan (Dunn et al., 2008).
To obtain the wasps used in the survival experiments, figs containing wasps were collected at dawn, within a few hours of when  (Table A3). The petri dishes were sealed and kept in growth chambers (Percival I-36LL and Percival Intellus) that mimicked natural environmental conditions for the wasps (12-h light/dark regime and a relative humidity of 85%) (Paton, 2020). The only thing that differed between the different treatments was the temperature. To further ensure identical conditions, temperature and humidity were confirmed using an independent thermometer and hygrometer that were regularly transferred between the chambers.

| Statistical analyses
To visualize differences in lifespan across treatments, survival curves for each treatment and wasp species were computed using packages "survival" (ver.  ) and were additionally potentially full sisters. We controlled for the variation between figs by including petri dish ID as a random factor in the survival model. We checked for all species if the random factor was significant by comparing the model with and without the random factor in an ANOVA; for all species, the random factor was significant (p < .001) except for Heterandrium (p = .97). The random factor was nevertheless included in all final models, as it corresponds to the experimental design. In the models for Pegoscapus hoffmeyeri, Tetrapus costaricanus, Pegoscapus gemellus, and Heterandrium, the highest temperature treatment (34°C resp. 36°C) had to be excluded from the analysis because there was not sufficient variation within these treatments.
In the survival models, we used 26°C as the baseline temperature as this is the temperature closest to the mean diurnal temperature except for the model of Pegoscapus tonduzi where a baseline temperature of 28°C was used due to a lack of enough variance in the 26°C treatment. Additionally, significant differences in lifespan between all pairwise temperature comparisons were investigated using Tukey contrasts with Bonferroni adjusted p-values, as implemented in the "multcomp" package (Bretz et al., 2010).

| Pollinator response to temperature increase
For all pollinator species, we found a significant effect of tem-    Exceptionally warm days, or even hours, would reduce wasps' lifespan dramatically. The median pollinator lifespan at 36°C was in our study reduced to merely 2-10 h (6%-19% of the baseline median lifespan), and in tropical Singapore to 1-4.5 h (4%-15% of baseline median lifespan) (Jevanandam et al., 2013). Pollinator fig wasps in temperate Australia, a more variable climate than the tropics, were slightly more tolerant to such high temperatures (50% median lifespan reduction at 35°C), but experienced a reduction to 14% median lifespan at 40°C (Sutton et al., 2018). Temperatures of 40°C already occur near the Australian study site approximately 5 days per year (Sutton et al., 2018). Extreme weather events such as these (36°C in tropics, 40°C in temperate regions) could both kill wasps while still inside the fig, and prevent dispersal of already emerged adults (Jevanandam et al., 2013;Sutton et al., 2018). Temperatures of 36°C are currently not occurring above the forest canopy in central Panama; the maximum measured temperature in 2002-2017 was 34.6°C (Paton, 2020). However, with a projected temperature increase of 4°C, temperatures above 36°C are expected to occur more frequently.
While all pollinator species showed a clear reduction in lifespan with increased temperatures, the reduction was particularly dramatic for those wasp species that had higher median lifespans at the   This may reflect density of the host fig species in a natural forest, but we currently do not have sufficient data to test this hypothesis. Within a genus, larger wasp species seem to have longer median lifespans than smaller wasp species (Herre, 1989;Jandér et al., 2016) but additional species would be needed to test this hypothesis.
The parasitic wasps also showed a clear reduction in lifespan with increasing temperatures ( Figure 5) Ghara & Borges, 2010). However, the difference in lifespan with and without food might not be dramatic.
Although our methods differ, therefore making direct comparison impossible, it seems that access to food may not dramatically prolong life for Idarnes, and our lifespan estimates for these parasites may be valid approximations.  (Mawdsley et al., 1998;Nason et al., 1996Nason et al., , 1998Todzia, 1986;  Jandér & Steidinger, 2017) and increasing inbreeding of the wasps (Herre, 1985;Herre et al., 2008;Molbo et al., 2004). Further adding to the problem, logging practices that reduce South American forest area with 2.6 million hectares per year cause forest fragmentation, thus further increasing distances between fig trees (Broadbent et al., 2008;FAO, 2020;Hansen et al., 2013;Mawdsley et al., 1998).
Logging also increases the local temperature by biomass removalthe air above logged areas can be 5-10°C hotter than nearby intact forest environment (Blonder et al., 2018). Forest fragmentation therefore not only increases distances between trees, but might also reduce wasp lifespan even further. Forest fragmentation in combination with global warming could therefore be devastating for the continued pollination of fig trees.
Highly specialized and obligate mutualisms are expected to be more vulnerable to the effects of rapid environmental change than relationships based on more generalist partners (Kiers et al., 2010;Vidal et al., 2021).

ACK N OWLED G M ENTS
We thank S. Paton

TA B L E A 1
Overview of the different species included in this study.

Tree species Figs (n)
Pollinator wasps (n) Note: The number of parasitic wasps indicated here are those of the genera Idarnes, Heterandrium, and Critogaster; additional parasitic wasps genera were present but not included in the study.

TA B L E A 2
Summary of the overall sample sizes in this study. See Table A3 for the number of figs (i.e., wasp cohorts) and individual wasps in each temperature treatment.